Review articleImaging: Gear up for mechano-immunology
Introduction
Cells have remarkable abilities to sense and respond to the mechanical properties of their environment [1]. In recent decades, immune cells were also found to be sensitive to the mechanical properties of their microenvironment, which can regulate the immune response in diverse processes, such as immune cell activation, pathogen clearance, antigen presentation, cytokine and antibody secretion, and memory responses [2], [3], [4], [5], [6], [7]. Cells of the immune system get activated when their membrane receptors bind to irritant antigens on the surface of antigen presenting cells (APCs). Both T and B lymphocyte signaling has been shown to be responsive to physical forces and mechanical cues and further regulate these immune cells to exert forces. Numerous studies have focused on how mechanical cues, such as the stiffness properties from the antigen presenting microenvironment, can regulate immune cell responses, and how immune cells can exert mechanical forces to accomplish specific immune functions, such as affinity discrimination, antigen extraction, migration, morphology changes, DNA regulation and target cell killing [2].
Mechanical forces regulate immune responses across a range of dimensions, from organs to molecules, from outside-in to inside-out, in the functions from passive response to mechanical cues to active response towards applying forces. At the organ level, mechanical force significantly modulates both inflammation and pain in a systematic way. For example, during the wound recovery process, mechanical stimulation through the incision region on the skin can transduce to T cells and induce T-cell-dependent Th2 cytokine production and chemokine signaling and further recruit systemic inflammatory cells, such as macrophages or monocytic fibroblast precursors, in response to mechanical loading [8]. Some studies of tumor growth and immune cell infiltration have found that the mechanical microenvironment changed dramatically during different stages of tumor progressions. Thus, changes in mechanical features could lead to a shortcoming of immune therapeutics by directly regulating immune cell responses or indirectly influencing inherent processes. For example, during the epithelial-to-mesenchymal transition process, enhancing cell survival through autophagy will in turn effect immune responses [9]. At the cell level, mechanical force studies in immune cells focus on 1) how the external mechanical information regulates the whole cell responses, such as activation, signaling transduction, cell motility, and gene profiles; 2) how the immune cell uses mechanical force from inside, for the cell adhesion, substrate discrimination, antigen uptake and internalization eventually to kill target cells [10]; 3) how immune cells react as a network to control the immune responses by transducing mechanical information through cell-cell interactions. At the molecular level, a mechanism for mechanical cues in immune cells is less established when it comes to the membrane-bound receptors, motor proteins, scaffold proteins and cytoskeleton. In T cells, T cell receptors (TCRs) mediate both recognition and T cell activation via their dimeric alpha beta, CD3 epsilon gamma, CD3 epsilon delta, and CD3 zeta subunits, of which the alpha beta chain is an anisotropic mechanosensor responsible for force responses [11], [12]. The actin cytoskeleton is essential for cell mechanics and has increasingly been implicated in the regulation of cell signaling. Cytoskeletal forces exerted by cells are likely mediated through mechanical modulation. The process of how the actin cytoskeleton couples to the activation of immune cells, in particular, B cells, was recently discussed by Pavel [13]. A better understanding of how mechanical cues regulate immune cells can benefit the design of vaccines for many diseases, like HIV [14]. Huse and Upadhyaya have recently reviewed the current understanding of how mechanical forces regulate membrane-bound receptor activation, cell migration, intracellular signaling, intercellular communication, tissue rearrangement, cell differentiation and immune responses, highlighting the biological ramifications of these effects in various immune cell types [2], [15]. All these studies employed fluorescent microscopy-based imaging techniques and methods geared towards measuring force associated profiles and molecular sensor systems. Moreover, multiple techniques can be combined to enhance the study of mechanical forces. For example, the biophysical techniques of micropipette, and micropillar-based traction force microscopy were used to reveal a striking correlation between the magnitude of force exertion across the synapse and the speed of perforin pore formation on the target cell, implying that force potentiates cytotoxicity by enhancing perforin activity [10]. Our review will focus on the imaging techniques used to elucidate mechanical force interactions in immune cells.
Section snippets
Imaging techniques for mechanical force studies in immune cells
Cells control mechanical forces and integrate these forces into tissues to form the final shape of an organism through a process called mechanotransduction, which ultimately affects morphology, migration, proliferation and apoptosis [16]. In the 1950s, mechanotransduction was first shown to affect cellular processes in a study where the cancer cells showed anchorage-independent growth on soft agar while non-cancer cells did not [17], [18]. This peculiar phenomenon of mechanosensing was
Fluorescent-microscopy-associated techniques for mechanical force studies in immune cells
The mechanical force associated profiles can be monitored by the techniques mentioned above, however, more information needs to be recorded and explored simultaneously in biological applications, such as the morphology changes, the size and tightness of adhesion, the activation of receptors, the signaling transductions and the location of molecules. Imaging studies have shown that the temporal and spatial information of the molecule of interests is valuable to interpret the molecular
Biophysical features of ligands regulate immune cell activations
Biophysical studies on immune cells include but not limited to the techniques mentioned above. Some other systems, do not directly measure or exert forces, but theoretically, they are associated with mechanical force studies.
Future work
Real-time imaging techniques at the micro and Nano-level are one of the breakthroughs of modern technology in this era. Nowadays, it facilitates not only the tissue-level visibility but also the detailed visualization of the single cell. With the advent of these modern technologies along with the biophysical methods mentioned here, we have made substantial progress in the immune cell studies. To further our understanding of complex immune studies, we need to continue developing new techniques
Acknowledgements
This work is supported by funds from the National Natural Science Foundation of China (81730043, 81422020 and 81621002), Ministry of Science and Technology of the People’s Republic of China (2014CB542500-03).
Conflict of interest disclosure
The authors declare no financial or commercial conflict of interest.
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